The present invention relates to a stent that is implanted in lumens such as the blood vessel, the bile duct, the trachea, the esophagus, the ureter, and the like so that it is used to improve a stenosed portion or a closed portion generated in the lumens.
To cure various diseases that are caused when the blood vessel or lumens in the human body are stenosed or closed, the stent which is a tubular medical appliance is implanted at the stenosed portion or the closed portion to expand them and secure the lumen thereof. Because the stent is inserted into the human from outside, its diameter is small. The stent is dilated or returned to its original shape to make its diameter large at the stenosed or closed portions to keep the dilated state of the lumen.
The stent is classified into a self-expandable stent and a balloon expandable stent, depending on the function and dilating mode thereof.
The balloon expandable stent which itself has no dilating function is inserted into a desired portion. Then, a balloon provided in the stent is inflated to dilate (plastically deform) the stent so that the stent is fixed to the inner surface of the desired lumen, with the stent in close contact therewith. That is, it is necessary to dilate the stent of this type in implanting it in the desired portion.
Fundamentally, the self-expandable stent is made of an elastic material. The final size of the self-expandable stent is set when it is expanded. In introducing the self-expandable stent into the human body, it is folded into a small size and put into a member (plastic tube in most cases) restricting its configuration. Then the member, namely, the tube is introduced into the human body. The self-expandable stent is discharged from the tube at the desired portion. The self-expandable stent dilates itself owing to its elasticity.
The dilating mode of the balloon expandable stent and that of the self-expandable stent are different from each other. The characteristic of the balloon expandable stent and that of the self-expandable stent are also different from each other. These two kinds of the stents have merits and demerits. The balloon expandable stent dilates in the form of a plastic deformation in conformity to the dilation of the balloon. Therefore the balloon expandable stent can be embedded in a curved blood vessel, with the balloon expandable stent curved plastically. However, in the case where the balloon expandable stent is embedded in a sublimis blood vessel (artery near the surface of human body such as carotid arteries, femoral artery, and the like), there is a fear that the balloon expandable stent is deformed plastically by an external force. Generally, embedded into such a portion is the self-expandable stent that is capable of returning to its original configuration by its elasticity, even though it is deformed by an external force applied thereto. The self-expandable stent has property of returning to its original configuration. In most cases, the stent is formed straight in its longitudinal direction. Thus even though the self-expandable stent is so configured that it can be curved at a light force, it will return to its original (straight) configuration in the human body. Therefore when the self-expandable stent is implanted in a curved blood vessel, the force of the self-expandable stent of returning to its original straight shape is always applied to both ends thereof.
The self-expandable stent is disclosed in U.S. Pat. No. 6,042,606 (WO99/16,387). The stent disclosed therein is formed straight in its longitudinal direction. Thus even though the self-expandable stent is so configured that it can be curved at a light force, it will return to its original (straight) configuration in the human body. Therefore when the self-expandable stent is implanted in a curved blood vessel, the force of the self-expandable stent of returning to its original straight shape is always applied to both ends thereof.